Summary of Senate Races 1992-2004
The data used for the Filibuster 2005 analysis comes from the Clerk of the U.S. House. You can download their election results data [HERE]. Please note that we've discovered the Clerk of the U.S. House incorrectly lists the 2004 Oregon Senate results for Senator Wyden - as their listed figure is double the actual 1,128,728 votes he received. We've also included below the Excel sheets containing our compiled U.S. Senate election results:

Summary of Individual Election Cycles
1992:
Democrats won 20 seats in the Senate, which constitutes roughly 59% of the total 34 seats up for election in this cycle, despite the fact that they only won roughly 49% of the popular vote.  This imbalance helped lead the Democrats to take a 10% over-representation in the Senate, due to the current winner-take-all plurality system.

Republicans, meanwhile, won 14 seats in the Senate, which constitutes roughly 41% of the total 34 seats up for election in this cycle, despite the fact that they won 44% of the popular vote.  This imbalance helped lead the Republicans to suffer from a 3% under-representation in the Senate.

1994:
Democrats won 14 seats in the Senate, which constitutes roughly 41% of the total 34 seats up for election in this cycle, despite the fact that they won 44% of the popular vote.  This imbalance led the Democrats to suffer from a 3% under-representation in the Senate.

Republicans, meanwhile, won 20 seats in the Senate, which constitutes roughly 59% of the total 34 seats up for election in this cycle, despite the fact that they only won 50% of the popular vote.  This imbalance led the Republicans to take an 11% over-representation in the Senate, due to the current winner-take-all plurality system. 

1996:
1996 marks perhaps the fairest year for partisan elections during this time span, as the numbers of seats attained by each party were the closest to achieving proportionality with the number of voters actually represented by each party. 

Democrats won 17 seats in the Senate, which constitutes 50% of the 34 seats available, which is fairly proportional when compared to the fact that they won 48% of the popular vote.  There was a slight 1% under-representation in the Senate.

Republicans, meanwhile, also won 17 seats in the Senate, which constitutes 50% of the total 34 seats up for election in this cycle. Again, this was a fairly proportional result, given that they won 49% of the popular vote.  There was a slight 2% under-representation in the Senate.   

1998:
Democrats won 18 seats in the Senate, which constitutes roughly 53% of the total 34 seats up for election in this cycle, despite the fact that they only won 49% of the popular vote.  This imbalance helped lead the Democrats to take a 4% over-representation in the Senate, due to the current winner-take-all plurality system.

Republicans, meanwhile, won 16 seats in the Senate, which constitutes 47% of the total 34 seats seats up for election in this cycle. This is fairly close to the 47% of the popular vote the party received in this cycle.  As a result, there were no imbalances in proportionality, and therefore no under or over-representation.

2000:
Democrats won 18 seats in the Senate, which constitutes roughly 53% of the total 34 seats up for election in this cycle, despite the fact that they only won 46% of the popular vote.  This imbalance led the Democrats to take a 7% over-representation in the Senate, due to the current winner-take-all plurality system.

Republicans, meanwhile, won 16 seats in the Senate, which constitutes 47% of the total 34 seats up for election in this cycle, despite the fact they won 46% of the popular vote.  This imbalance led Republicans to suffer from a 1% under-representation in the Senate.  It is more important to note, however, that while both the Democrats and Republicans won the same percentage of the popular vote, the Democrats got dramatically more representation in the Senate, 8% more, to be exact. 

2002:
Democrats won 12 seats in the Senate, which constitutes roughly 35% of the total 34 seats up for election in this cycle, despite the fact that they won 46% of the popular vote.  This imbalance led the Democrats to suffer from a 9% under-representation in the Senate.

Republicans, meanwhile, won 22 seats in the Senate, which constitutes 65% of the total 34 seats up for election in this cycle, despite the fact that they only won 50% of the popular vote.  This imbalance led the Republicans to enjoy a staggering 15% over-representation in the Senate, due to the current winner-takes-all plurality system. 

2004:
2004 marks the clearest example of the classic failure of the plurality system in adhering to an ideal of fairness and proportionality in representation.  During the time period studied, 2004 marks the one election cycle when the party that won the majority of the popular vote actually won aminority of the Senate seats available.  Although during other years, there had been over and under representation, they all occurred under a framework where at least the party who won the majority of popular votes received the majority of Senate seats.  Unfortunately, in 2004, this was not the case.

Democrats won 15 seats in the Senate, which constitutes roughly 44% of the total 34 seats available, despite the fact that they had won a majority of 51% of the popular vote.  This imbalance led the Democrats to suffer from a mild 7% under-representation in the Senate, but more importantly, this 7% under-representation occurred when the Republicans didn’t even attain a majority of the popular vote. 

Republicans, meanwhile, won 19 seats in the Senate, which constitutes 56% of the total 34 seats available, despite the fact that they had won a dismal minority of the 45% of the popular vote.  This imbalance led the Republicans to enjoy an 11% over-representation in the Senate, a figure further tainted under the consideration that the Republicans failed to secure the majority of the popular vote.   

Note: The term popular vote is used to mean the total number of votes cast in all Senate races, including those cast for losing candidates.